A conventional color image forming apparatus using an electrophotography method normally adopts the following system. That is, a single photosensitive body undergoes development of colors using a plurality of developing units, and an exposure-development-transfer process is repeated a plurality of times, thus overlaying and forming color images on a single transfer sheet. The color images are fixed to obtain a full-color image.
According to this system, an image forming process must be repeated three or four times (if black is used), and it takes much time. As a method of removing such shortcomings, the following method is known. That is, a plurality of photosensitive bodies are used, and toner images obtained for respective colors are overlaid in turn on a transfer sheet to obtain a full-color print via a single paper feed operation. According to this method, the throughput can be greatly improved. On the other hand, a shifting amount of color problem occurs due to shifting amounts of color of respective colors on a transfer sheet resulting from errors of the accuracy of position and diameters of photosensitive bodies, and errors of accuracy of position of optical systems, and it is difficult to obtain a high-quality full-color image.
As a method for preventing this shifting amount of color, for example, the following method may be used. That is, a test toner image on a transfer sheet or a transfer belt which forms a transfer unit is detected, and the optical path of each optical system is compensated or the image write start position of each color is be compensated based on the detection result (see Japanese Patent Laid-Open No. 64-40956). However, this method poses the following problems.
First, in order to compensate the optical path of the optical system, a compensation optical system including a light source and f-θ lens, a mirror in the optical path, and the like must be mechanically moved to adjust the position of the test toner image. For this purpose, high-precision movable members are required, resulting in high cost. Furthermore, since it takes much time until compensation is completed, it is impossible to frequently perform compensation. However, an optical path length difference may change along with an elapse of time due to temperature rise of mechanical components. In such case, it becomes difficult to prevent shifting amount of color by compensating the optical path of the optical system.
Second, in order to compensate the image write start position, it is possible to conduct shifting amount of color compensation of the left end and upper left portion but it is impossible to compensate for the tilt of the optical system and to compensate for any magnification errors due to the optical path length difference.
The following arrangement has also been proposed (see Japanese Patent Laid-Open No. 8-85237). That is, the output coordinate positions of image data of respective colors are converted into those free from any registration errors. After that, based on the converted image data of respective colors, the positions of modulated light beams are compensated by an amount less than the minimum dot unit of a color signal. Since the output coordinate position of image data for each color is compensated with respect to an image that has undergone half-tone processing, the reproducibility of half-tone dots of a half-tone image deteriorates. As a result, color inconsistency may occur and moire may become obvious.
FIG. 4 shows such example. An input image 101 has constant color density values. Upon printing an image 102 obtained by applying arbitrary compensation shifting amount of color to the input image 101 in practice, since the image color density value and a toner color density for that image color density value do not have a linear relationship, although the input image 101 has constant color density values, an image whose color density values are not constant is printed. When such non-uniform color density values are periodically repeated, moieé becomes obvious, and a high-quality color image cannot be obtained.
Furthermore, since the print speed increases, a photosensitive body scanned by a laser beam does not stand still while the laser beam is being scanned, and moves by a predetermined amount according to the print conditions during a laser scanning time period. If the scan directions of lasers of respective colors agree with each other, the inclination of a scan line due to this moving amount does not pose any problems. However, the inclination of the scan line may cause deterioration of image quality such as color inconsistency depending on the moving amount of the photosensitive body between colors for which scans start from opposite directions. Also, this moving amount varies depending on conditions such as a print medium and the like and cannot be compensated by simple processing.
As described above, it is conventionally difficult to provide an image which is free from any moire that may be caused by compensation of shifting amount of color and suffers less deterioration of image quality.